While there are no fundamental reasons for any video mode to not work, most of the DisplayPort to HDMI adapters that are currently on the market do not support the latest standard versions of DisplayPort and/or HDMI, so when a very high performance mode is desired, it might not work.
However, the main use of adapters is when you travel and you find in your temporary office an HDMI-only monitor, or when you must use a meeting room projector. Such monitors or projectors seldom support high performance video modes.
A problem is also that many video cards that do support HDMI 2.1 only support display port DisplayPort 1.4 which has less bandwith. This makes the sitution with the open source AMD drivers even more annoying because even with an active adapter that supports all the required features (which most don't) you can't get the full HDMI 2.1 resolutions/refresh rates that way.
However, the main use of adapters is when you travel and you find in your temporary office an HDMI-only monitor, or when you must use a meeting room projector. Such monitors or projectors seldom support high performance video modes.